Solid tumors are complex tissues containing both neoplastic tumor cells and non-neoplastic cell types (e.g., vascular endothelial cells (EC), fibroblasts and macrophages) and the interactions between these cells can regulate tumor progression. For example, tumor cells produce multiple proteins that act in a paracrine manner to influence neighboring EC. Some of the proteins induce angiogenesis, which is crucial for primary tumor growth and metastasis, while others trigger immune system cell infiltration, which has both pro- and anti-tumorigenic effects. In this proposal, we describe experiments to investigate whether the cytokine tumor necrosis factor-like weak inducer of apoptosis (TWEAK) may play a multifunctional role in the tumor microenvironment. TWEAK expression has been detected in human tumor cells and tumor-associated macrophages, and these cells are likely the predominant sources of this cytokine in tumor tissue. TWEAK, acting via binding to the fibroblast growth factor-inducible 14 (Fn14) cell surface receptor, is an angiogenic factor in vivo. Additionally, since TWEAK treatment of EC activates the nuclear factor-?B (NF-?B) signaling pathway and promotes pro-inflammatory molecule production, this cytokine may also be a pro-inflammatory factor in vivo. In consideration of these and other findings, we hypothesize that the TWEAK-Fn14 signaling system may play an important role in both tumor angiogenesis and inflammation. Specifically, we propose that TWEAK acts at two phases of tumor development. In phase 1, tumor cell-derived TWEAK functions primarily as a paracrine regulator of EC activation, triggering both tumor angiogenesis and immune cell infiltration. In phase 2, immune system cells within the tumor stroma, in particular blood monocyte-derived macrophages, become an important secondary source of this cytokine. This TWEAK protein can also act on EC, thereby further contributing to tumor angiogenesis and persistent monocyte infiltration. TWEAK released from both cellular sources may also act on the tumor cells themselves, and all of these interactions will ultimately contribute to tumor growth and metastasis. Four Specific Aims have been formulated to test our hypothesis. These Aims are: (1) To determine if TWEAK can induce pro-angiogenic and pro-inflammatory cellular responses in vitro, and to investigate if TWEAK activity is dependent on NF-?B pathway activation, (2) To determine whether TWEAK delivered into the tumor microenvironment can stimulate angiogenesis, inflammation, and tumor growth, and to investigate if these effects are dependent on NF-?B activation, (3) To determine whether TWEAK present in the tumor microenvironment promotes tumor growth, at least in part, by acting on non-neoplastic host cells and to determine if TWEAK produced by host immune cells contributes to tumor growth, and (4) To determine if TWEAK activity contributes to tumorigenesis in a spontaneous mammary cancer model and to investigate whether TWEAK antagonist administration can inhibit tumor growth in vivo. The proposed studies should provide important, novel information on the role of TWEAK and Fn14 in the tumor microenvironment and will provide insight into whether TWEAK is a potential therapeutic target for human cancer. PUBLIC HEALTH RELEVANCE: Cancer is the second leading cause of mortality in the USA. Solid tumors are complex tissues containing both transformed tumor cells and normal cell types such as the vascular endothelial cells that line blood vessels and blood-derived immune system cells that infiltrate the tumor tissue. Previous studies have revealed that interactions between these different cell types can regulate tumor progression. In this application, we propose to investigate whether a particular cytokine, named TWEAK, is produced by multiple cell types in the tumor microenvironment and acts as an important stimulatory molecule contributing to blood vessel formation, tumor inflammation, and ultimately tumor growth.